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The present invention relates to a through-one-end water collection type
hollow fiber membrane and a method for manufacturing the same. According
to the present invention, an internal sealing part is formed in the
hollow part of the membrane at its free end. The internal sealing part
supports an external sealing part surrounding the external surface of the
free end such that the durability of the whole sealing part might be
improved.

Inventors:

RYU; JaeHee; (Seoul, KR); LEE; Moo-Seok; (Seoul, KR)

Assignee:

Kolon Industries, Inc.Kwacheon-siKR

Serial No.:

091361

Series Code:

13

Filed:

April 21, 2011

Current U.S. Class:

210/500.23

Class at Publication:

210/500.23

International Class:

B01D 69/08 20060101 B01D069/08

Foreign Application Data

Date

Code

Application Number

Aug 7, 2007

KR

10-2007-0078829

Claims

1. A hollow fiber membrane comprising: an external sealing part
surrounding an external surface of a first end of the hollow fiber
membrane; and an internal sealing part in a hollow part of the hollow
fiber membrane at the first end.

2. The hollow fiber membrane of claim 1, wherein the length of the
internal sealing part is 1 to 200% of the length of the external sealing
part, the length of the internal and external sealing parts being
parallel to a longitudinal direction of the hollow fiber membrane.

3. The hollow fiber membrane of claim 1, wherein the external and
internal sealing parts are formed of a same material.

4. The hollow fiber membrane of claim 3, wherein the external and
internal sealing parts each comprise a polyurethane, a silicone, a
heat-cured polymer, or a UV-cured polymer.

5. The hollow fiber membrane of claim 1, wherein a cross section of the
external sealing part perpendicular to the longitudinal direction of the
hollow fiber membrane becomes smaller as farther from a second end of the
hollow fiber membrane opposite to the first end.

Description

[0001] This application is a divisional application of U.S. application
Ser. No. 12/186,686 filed on Aug. 6, 2008. This application claims the
benefit of Korean Patent Application No. 10-2007-0078829 filed on Aug. 7,
2007, which is hereby incorporated by reference for all purposes as if
fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a hollow fiber membrane and a
method for manufacturing the same, and more particularly to a
through-one-end water collection type hollow fiber membrane having a free
end which is not fixed to a header whereby permitting relatively free
movement of the membrane when submerged in feed water to be treated.

[0004] 2. Discussion of the Related Art

[0005] A separation method using a membrane has lots of advantages
compared to the method based on a phase inversion or heating. Among the
advantages is high reliability of water treatment since the water purity
required may be easily and stably satisfied by adjusting the size of the
pores of a membrane. Further, a membrane can be used with microorganism
which is useful for separation process but may be adversely affected by
heat.

[0006] A membrane for separation includes a flat sheet membrane and a
hollow fiber membrane.

[0007] A hollow fiber membrane module carries out a separation process
using a bundle of hollow fiber membranes. Typically, a hollow fiber
membrane module has been widely used in the field of microfiltration and
ultrafiltration for obtaining axenic water, drinking water, super pure
water, and so on. Recently, however, application of the hollow fiber
membrane module is being expanded to include wastewater treatment,
solid-liquid separation in a septic tank, removal of suspended solid (SS)
from industrial wastewater, filtration of river, filtration of industrial
water, and filtration of swimming pool water.

[0008] Among such hollow fiber membrane modules is a submerged hollow
fiber membrane module which is submerged into a tank of feed water to be
treated. Negative pressure is applied to the internal parts of the hollow
fiber membranes such that only fluid passes through the walls of the
membranes and solids and sludge are rejected and accumulate in the tank.
A submerged hollow fiber membrane module is used mainly in the form of a
cassette having a plurality of modules combined to a frame. A submerged
hollow fiber membrane module is advantageous in that the manufacturing
cost is relatively low and that the installation and maintenance cost may
be reduced since a facility for circulating fluid is not required.

[0009] When a submerged hollow fiber membrane module is used to treat
wastewater, the solids in the wastewater fouls the membrane causing the
permeability of the membrane to be declined as the wastewater is treated.
The solids may be present in the feed water in a variety of forms which
contribute to fouling in different ways. To counter the different types
of fouling, many different types of cleaning regimens may be required.

[0010] Such cleaning may be classified into maintenance cleaning and
recovery cleaning according to the cleaning purposes.

[0011] The maintenance cleaning is a cleaning performed while the water
treatment is carried out by the hollow fiber membrane module or a
cleaning performed only for a short time after the water treatment is
stopped. The main purpose of the maintenance cleaning is to maintain the
permeability of the membranes in good status. The maintenance cleaning is
generally carried out by physical cleaning The most frequently used
methods of physical cleaning are backwashing and aeration.

[0012] In backwashing, permeation through the membranes is stopped
momentarily. Air or water flows through the membranes in a reverse
direction to physically push solids off of the membranes. On the other
hand, in aeration, bubbles are produced in the tank water below the
membranes. As the bubbles rise, they agitate or scrub the membrane and
thereby remove the solids while creating an air lift effect and
circulation of the tank water to carry the solids away from the
membranes.

[0013] Based on the water collection type, a submerged hollow fiber
membrane module may be classified into a through-both-ends water
collection type and a through-one-end water collection type. According to
a through-both-ends water collection type, the permeate obtained inside
each hollow fiber membrane is collected through both ends of the
membranes. On the other hand, the permeate is collected through only one
end of each membrane in a through-one-end water collection type.

[0014] In case of a through-both-ends water collection type hollow fiber
membrane module, two ends of the membrane are fixed to two headers
respectively. Each header has a permeate collecting space therein with
which the membrane is in fluid communication. When performing maintenance
cleaning by means of aeration, upward movement of bubbles from a aeration
tube are interrupted by the headers especially when the hollow fiber
membrane module is a vertical type. An upper header has the effect of
displacing the rising bubbles towards the outside of the membrane bundle.
Thus, effective aeration is no longer guaranteed in the upper region of
the membrane. As a consequence, relatively severe fouling occurs in the
upper region of the membrane bundle.

[0015] On the other hand, in case of a through-one-end water collection
type hollow fiber membrane module, only one end of the membrane is fixed
to a header and the other end, a free end, is free to move. Thus,
interruption of water flow caused by rising bubbles emitted from the
aeration tube is remarkably reduced, and thus a vertical hollow fiber
membrane of through-one-end water collection type may guarantee more
effective aeration over the entire length of the membrane than a vertical
hollow fiber membrane of through-both-ends water collection type. For
this reason, a hollow fiber membrane of through-one-end water collection
type has been actively studied.

[0016] Since the free ends of the membranes in the through-one-end water
collection type hollow fiber membrane module are not fixed to a header,
every each of the membranes must be sealed at their free ends. Since the
durability of sealing part is relatively weak, the durability of the
whole hollow fiber membrane depends on how to seal the free ends of the
membranes.

[0017] A method of sealing a hollow fiber membrane at its free end
comprises coating the free end with a sealant of same polymer, e.g.,
polyethersulfone (PES), as that of the membrane, and curing the sealant.
FIG. 1 shows a cross section of a hollow fiber membrane sealed with this
method. As shown in FIG. 1, the sealing part 110 exists only on the
external surface of the free end of the hollow fiber membrane 100. Thus,
the sealing part 110 is vulnerable and might be easily stripped off from
the membrane 100 causing leakage, which requires replacement of the
impaired membrane 100 and thus increases the maintenance cost.

SUMMARY OF THE INVENTION

[0018] Accordingly, the present invention is directed to a hollow fiber
membrane and a method for manufacturing the same that substantially
obviates one or more of the problems due to limitations and disadvantages
of the related art.

[0019] An advantage of the present invention is to provide a hollow fiber
membrane used for a through-one-end water collection type hollow fiber
membrane module and a method for manufacturing the same, wherein the
hollow fiber membrane is provided with a sealing part of improved
durability at its free end.

[0020] Another advantage of the present invention is to provide a
composite hollow fiber membrane sealed at its free end without impairing
the coating layer thereof and a method for manufacturing the same,
wherein the sealing part has high durability.

[0021] Further another advantage of the present invention is to provide a
hollow fiber membrane having a free end surrounded by a sealing part a
portion of which is inserted into the hollow part of the membrane in
sufficient length, and a method for manufacturing the same.

[0022] Still further another advantage of the present invention is to
provide a hollow fiber membrane sealed with a sealant effectively at its
free end, and a method for manufacturing the same.

[0023] Additional features and advantages of the invention will be set
forth in the description which follows, and in part will be apparent from
the description, or may be learned by practice of the invention. The
objectives and other advantages of the invention will be realized and
attained by the structure particularly pointed out in the written
description and claims hereof as well as the appended drawings.

[0024] To achieve these and other advantages and in accordance with the
purpose of the present invention, as embodied and broadly described,
there is provided a hollow fiber membrane comprising an external sealing
part surrounding an external surface of a first end of the hollow fiber
membrane, and an internal sealing part in a hollow part of the hollow
fiber membrane at the first end.

[0025] In another aspect of the present invention, there is provided a
method for manufacturing a hollow fiber membrane, the method comprising
forming an internal sealing part in a hollow part of the hollow fiber
membrane at an end of the hollow fiber membrane, and forming an external
sealing part surrounding an external surface of the end of the hollow
fiber membrane.

[0026] It is to be understood that both the foregoing general description
and the following detailed description are exemplary and explanatory and
are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWING

[0027] The accompanying drawings, which are included to provide a further
understanding of the invention and are incorporated in and constitute a
part of this specification, illustrate embodiments of the invention and
together with the description serve to explain the principles of the
invention.

[0031] FIG. 3 is a cross sectional view illustrating a hollow fiber
membrane according to the first embodiment of the present invention;

[0032] FIG. 4 is a cross sectional view illustrating a hollow fiber
membrane according to the second embodiment of the present invention;

[0033] FIG. 5 is a cross sectional view illustrating a hollow fiber
membrane according to the third embodiment of the present invention; and

[0034] FIG. 6 is a cross sectional view illustrating a hollow fiber
membrane according to the fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0035] Reference will now be made in detail to an embodiment of the
present invention, example of which is illustrated in the accompanying
drawings.

[0036] It will be apparent to those skilled in the art that various
modifications and variation can be made in the present invention without
departing from the spirit or scope of the invention. Thus, it is intended
that the present invention cover the modifications and variations of this
invention provided they come within the scope of the appended claims and
their equivalents.

[0038] A through-one-end water collection type hollow fiber membrane
module, as shown in FIG. 2, has a bundle of hollow fiber membranes 100.
An end of each membrane 100 is potted into a header 200 inside which a
water collecting space 210 is formed. The end of the membrane 100 potted
into the header 200 is open and thus in fluid communication with the
water collecting space 210. Thus, the permeate which flows into the
hollow part through the membrane 100 can flow to the water collecting
space 210. The other end, a free end, of the membrane 100 is not fixed to
anywhere and is sealed for the feed water not to be able to flow into the
hollow part through the open free end.

[0039] The hollow fiber membrane 100 may be a porous single membrane
comprising a polymer such as polyethersulfone(PES), polysulfone(PS), and
polyvinylidene difluoride(PVDF) or a composite membrane comprising a
tubular braid and a polymer coating layer thereon.

[0040] FIGS. 3 to 5 are cross sectional views illustrating hollow fiber
membranes according to the first, second, and third embodiments of the
present invention respectively.

[0041] As shown in FIGS. 3 to 5, the hollow fiber membrane 100 of the
present invention has a free end which is not fixed to anywhere. The free
end is closed by sealing part 120a, 120b, and 120c. The sealing part
120a, 120b, and 120c comprises an external sealing part surrounding the
external surface of the free end, and an internal sealing part in the
hollow part of the membrane 100 at the free end. The internal sealing
part supports the external sealing part thereby improving the durability
of the whole sealing part 120a, 120b, and 120c.

[0042] According to an illustrative embodiment of the present invention,
the length l.sub.1 of the internal sealing part is 1 to 200% of the
length l.sub.2 of the external sealing part. The length l.sub.1 and
l.sub.2 of the internal and external sealing parts are parallel to a
longitudinal direction of the hollow fiber membrane 100. If the length
l.sub.1 of the internal sealing part is less than 1% of the length
l.sub.2 of the external sealing part, the internal sealing part can
hardly support the external sealing part. On the contrary, if the length
l.sub.1 of the internal sealing part is more than 200% of the length
l.sub.2 of the external sealing part, it takes too much time to cure the
internal sealing part, too much sealant is consumed, and the durability
of the sealing part 120a, 120b, and 120c may be reduced due to the low
viscosity of the sealant when the internal and external sealing parts are
formed of the same material.

[0043] The length l.sub.1 of the internal sealing part of the embodiment
shown in FIG. 3 is shorter than the length l.sub.2 of the external
sealing part. On the other hand, the length l.sub.1 of the internal
sealing part of the embodiment shown in FIG. 4 is longer than the length
l.sub.2 of the external sealing part.

[0044] In cases of the hollow fiber membranes 100 according to the first,
second, and third embodiments of the present invention respectively shown
in FIGS. 3 to 5, the external and internal sealing parts may be formed of
the same sealant substantially simultaneously. That is, the step for
forming the internal sealing part in the hollow part of the membrane 100
at the free end and the step for forming the external sealing part
surrounding the external surface of the free end may be performed at the
same time. A method for forming the external and internal sealing parts
simultaneously will be illustrated in detail hereinafter.

[0045] First, the viscosity of the sealant is adjusted such that the
sealant might be effectively drawn into the hollow part of the membrane
100 by capillary phenomenon. The viscosity of the sealant depends on the
temperature of the sealant and the characteristics of the material
itself. According to an illustrative embodiment of the present invention,
the sealing part 120a, 120b, and 120c is formed of polyurethane. When
polyurethane having viscosity of about 1,000 to 3,000 cps is used at
ambient temperature, the length l.sub.1 of the internal sealing part will
be shorter than the length l.sub.2 of the external sealing part. On the
contrary, when polyurethane having viscosity of about 100 to 500 cps is
used at a temperature above 30.degree. C., the length l.sub.1 of the
internal sealing part will be longer than the length l.sub.2 of the
external sealing part.

[0046] When the hollow fiber membrane 100 is a composite membrane
comprising a tubular braid as a reinforcing structure and a polymer
coating layer formed thereon, it might be advantageous that the sealing
part 120a, 120b, and 120c and the coating layer are formed of materials
different from each other for the reason described below.

[0047] When the hollow fiber membrane 100 is submerged into a sealant of
same polymer, e.g., polyethersulfone (PES), as that of the coating layer
of the membrane to form the sealing part 120a, 120b, and 120c at the free
end, the PES solution should have viscosity low enough for the solution
to be drawn into the hollow part of the membrane 100 at the free end. The
PES solution may be prepared by dissolving PES into its good solvent such
as DMAc, DMF, NMP, and so on. The amount of the solvent should be
increased to lower the viscosity of the PES solution. When the viscosity
of the solution becomes too low, however, some problems as follow might
occur.

[0048] First, the solvent of the PES solution may dissolve the PES coating
layer as well thereby impairing the membrane 100. Second, the thickness
of the sealing part, especially the portion corresponding to the corner
of the free end, cannot but be reduced due to the low viscosity of the
PES solution, and thus the possibility of the impairment of the membrane
and leakage at that point increases.

[0049] Hence, it may be advantageous that the sealing part 120a, 120b, and
120c comprises a material, e.g., polyurethane, silicone, heat-cured
polymer, or UV-cured epoxy polymer, which is different from that of the
coating layer of the membrane 100.

[0050] After the viscosity of the sealant is adjusted, a mold having an
inside space of a predetermined shape is filled with the
viscosity-adjusted sealant. In cases of the first and second embodiments
of the present invention as respectively shown in FIG. 3 and FIG. 4, the
predetermined shape of the inside space of the mold is a streamlined
shape, and thus the sealant cured in the mold would also have the
streamlined shape and could hardly be pulled out from the mold. Thus, if
having a inside space of a streamlined shape, the mold had better be
formed of a material which can be easily removed, e.g., paraffin.

[0051] In case of the third embodiment of the present invention as shown
in FIG. 5, the cross section of the external sealing part perpendicular
to the longitudinal direction of the hollow fiber membrane 100 becomes
smaller as farther from the other end of the hollow fiber membrane 100
opposite to the free end. Thus, the cured sealant of the third embodiment
of the present invention can be easily pulled out from the mold and the
mold need not be removed. Therefore, the third embodiment of the present
invention has an advantage in that there is no limitation on the material
forming the mold.

[0052] After the inside space of the mold is filled with the
viscosity-adjusted sealant, the free end of the hollow fiber membrane 100
is submerged into the viscosity-adjusted sealant in the mold. By
controlling the submerging time, the length by which the sealant is drawn
into the hollow part of the membrane 100 can be adjusted. Subsequently,
the sealing part 120a, 120b, and 120c is formed by curing the sealant
with heat or UV irradiation while the free end of the hollow fiber
membrane 100 is submerged in the sealant.

[0053] After the sealant is cured, as mentioned above, the molds of the
first and second embodiments of the present invention as illustrated in
FIG. 3 and FIG. 4 are removed. For example, if the mold is made of
paraffin, the mold can be removed by applying heat to the mold. On the
other hand, in the third embodiment of the present invention as
illustrated in FIG. 5, the sealing part 120c can be compulsorily pulled
out from the mold since the cross section of the sealing part 120c
perpendicular to the longitudinal direction of the hollow fiber membrane
100 becomes smaller as farther from the other end of the hollow fiber
membrane 100 opposite to the free end.

[0054] Referring to FIG. 6, a hollow fiber membrane and a method for
manufacturing the same according to the fourth embodiment of the present
invention will be described below.

[0055] A hollow fiber membrane 100 according to the fourth embodiment of
the present invention comprises an internal sealing part 130 in the
hollow part of the membrane 100 at the free end and an external sealing
part 140 surrounding the external surface of the free end. The internal
and external sealing parts 130 and 140 are formed of materials different
from each other.

[0056] An exemplary method for manufacturing the hollow fiber membrane 100
according to the fourth embodiment of the present invention will be
described below.

[0057] The first sealant is compulsorily injected into the hollow part of
the membrane 100 at the free end by means of an injector to form the
internal sealing part 130. Then, the end of the hollow fiber membrane 100
having the first sealant thereinside is submerged into the second sealant
to form the external sealing part 140.

[0058] Contrary to the first to third embodiments of the present invention
in which the sealant is drawn into the hollow part by capillary
phenomenon, according to the fourth embodiment of the present invention,
the first sealant is compulsorily injected into the hollow part to form
the internal sealing part 130, and can be put into the hollow part
without contacting with the external surface of the hollow fiber membrane
100. Thus, when the membrane 100 is a composite membrane, the first
sealant does not come into contact with the coating layer of the membrane
100 thereby avoiding the problem that the coating layer dissolves in the
solvent of the first sealant even if the material of the coating layer is
used for the first sealant. As a consequence, the fourth embodiment of
the present invention is advantageous in that the internal sealing part
130 can be formed of the same material as that of the coating layer and a
sealant of high viscosity can be used as the second sealant since the
external sealing part 140 is formed after the internal sealing part 130
is formed.

[0059] Optionally, the external sealing part 140 may be made of a material
different from that of the coating layer of the composite membrane 100
lest the coating layer should dissolve in the solvent of the second
sealant.

[0060] As an exemplary embodiment of the present invention, the external
sealing part 140 comprises polyurethane, silicone, heat-cured polymer, or
UV-cured polymer, and the internal sealing part 130 comprises the same
material as that of the coating layer, e.g., polyethersulfone(PES),
polysulfone(PS), or polyvinylidene difluoride(PVDF).

[0061] According to the hollow fiber membrane and method for manufacturing
the same of the present invention as above, a sealant can be easily put
into the hollow part of the through-one-end water collection type hollow
fiber membrane at its free end to form the internal sealing part without
impairing the coating layer of the membrane, and the internal sealing
part supports the external sealing part thereby improving the durability
of the whole sealing part.

[0062] Hence, the hollow fiber membrane and method for manufacturing the
same of the present invention can prevent the impairment of the sealing
part that may occur during module operation, thereby guaranteeing more
stable water treatment and cutting down the maintenance fee.

[0063] Although the hollow fiber membrane of the present invention is
described as one used in the field of the water treatment, it will be
apparent to those skilled in the art that it may also be applied to other
area where a hollow fiber membrane having at least one end sealed can be
used.